Answer:
An Era is usually defined as a major subdivision of geological Eon. It represents a period of long time, in terms of millions of years. There are 3 Eras of the Phanerozoic eon, according to the geological time scale, they are the Paleozoic, the Mesozoic and the Cenozoic. All these 3 Eras are comprised of many sequential arrangement of major and minor events soon after the formation of earth.
This sequential arrangement of events shows the progressive evolution from the unicellular organisms to the multi-cellular and more complex organisms. These eras are comprised of various fossils and fossil assemblages, which are explored in the recent times. The study of these fossils helped in the understanding and evolutionary pattern of how life evolved on earth.